I don't know about the Storybook breakfast, but for Mothers Day breakfast at Crystal Palace I was asked for one. I think it is because they must be in such demand that they want to assure people will come. Some people make ps for places and never show up for one reason or another, but it's a table that could have been filled if they would have called and cancelled. I think that by taking a credit card it either guarantee's Disney that either you will show up or you will cancel.
I know that I am rambling and probably not making much sense, but I hope you get get the gist of what I am saying.